Planet Facts: Interesting Facts about the Eight Planets Our solar system has eight planets: Mercury , Venus , Earth , Mars , Jupiter , Saturn , Uranus and Neptune . With the exception of Uranus and Neptune, each of these planets can be seen unaided. All eight planets can be see through the use of an inexpensive amateur telescope or binoculars. Order of the planets Order of the eight planets from left: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune The order of planets from closest to farthest from the Sun are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. The largest planet is Jupiter, followed by Saturn, Uranus, Neptune, Earth, Venus, Mars and, the smallest planet, Mercury. If you include dwarf planets as well, the planets in order becomes Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, Pluto, Haumea, Makemake and Eris being the furthest from the Sun. Distance of the planets from the Sun Planet Neptune What is a planet? The answer to this question is a highly controversial one. This has not always been the case, though.

In fact, before 1978 the definition of a “planet” was not really necessary. Until that time a planet simply meant a body in orbit around the Sun, that reflected sunlight, and was not a planetary moon, asteroid , or comet . However, with the discovery of Pluto’s moon Charon in 1978 scientists were able to calculate Pluto’s mass much more accurately than ever before and soon realized that it was much smaller than they had previously believed. At a tiny fraction of the mass of Mercury , Pluto was clearly a body much smaller than any other planet. This discovery led some to question whether Pluto was actually a planet or some other type of object. In the 1990s and early 2000s the discovery of several objects in the outer solar system similar in size to Pluto made it all but necessary to come to a definitive definition of a planet. Such a definition was needed to separate those types of objects like Pluto into a distinct class, otherwise all of the newly found objects would have to be called planets as well.

In response to this uncertainty, the International Astronomical Union (IAU), the official governing body for matters concerning naming astronomical objects, came to a definition of the term “planet.” According to the IAU, a planet is a celestial body that meets the following criteria: is in orbit around the Sun, has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and has cleared the neighbourhood around its orbit. How many planets are in the Solar System? According to the IAU’s definition for planet above, there are 8 known planets in the Solar System: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Pluto is no longer considered a planet under the IAU definition. Are there any undiscovered planets in the solar system? There have been several additional planets hypothesized throughout history; however, none of these planets has ever been found. The most recent of these theorized planets was Planet X, a supposedly giant planet used to explain the deviations from the predicted orbits of Uranus and Neptune.

Although Planet X has never been found and scientists are still unable to satisfactorily explain the uranian and neptunian orbital discrepancies, the scientific community has almost unanimously come to the conclusion that Planet X does not exist. It is highly unlikely that there are any planets beyond the orbit of Pluto. Most Popular The Planets Overview Bibliography The Planets The Solar System as we know it today contains 8 planets - the four rocky inner planets of Mercury, Venus, Earth and Mars, and then the four gas giants, comprising Jupiter, Saturn, Uranus and Neptune, with these two distinct groups seperated by the Asteroid Belt. There are also 3 known dwarf planets - objects not quite large enough to be considered proper planets, but too big to be thought of as mere asteroids or rubble. These are Ceres, Pluto and Eris (more on them below). Differences between inner & outer planets In the Solar System, we have two groups of planets - the outer gas giants and the inner rocky ones. The rocky ones are the first four, which are much smaller and denser than the outer four, containing a much higher percentage of metal elements, and less gases (Mercury and Mars do not in fact have any atmosphere to speak of).

Even Venus' thick, scorching atmosphere only accounts for a small fraction of its overall composition. The other four are instead a lot larger and have very dense atmospheres with (relatively) small cores. The reason for this state of affairs, put simply, is that when the Solar System was born, most of the gases near the Sun were soon either vapourised or blown away by the Solar wind so that only heavy metal & rock materials were left to form the inner planets. Further away though there was plenty of gas and this accumulated together to form behemoths the size of Saturn and Jupiter. Planets give out heat too... In the outer Solar System, the planets recieve much less light and heat from the Sun, as they are much further away, and the Solar radiation travelling outwards behaves according to an inverse square law. It is thus much easier to detect the small amounts of heat and light given out by the planets themselves. All planets give out energy to some degree; the terrestrial ones by slow radioactive decay of isotopes formed since the planet's formation, the gas giants emit heat left over from the planets' final phases of contraction when they were formed. With the exception of Uranus, the gas giants emit more heat than the terrestrial planets.

Inner Planets The inner planets, Mercury, Venus, Earth and Mars, can be (very) roughly split into 2 sub-groups. The first and last of the inner planets, Mercury and Mars respectively, are both comparatively small (Mercury has a diameter of 4,800km and Mars of 6,700km compared to Earth's 12,800km) and have no atmospheres to speak of. The middle two planets, Venus and Earth on the other hand, are very similar in size (Venus has a diameter of 12,100km to Earth's 12,800km) and have a similar composition, mass and density. However, Venus' position slightly closer to the Sun has resulted in a 'runaway greenhouse effect' creating a scorching, styfling inferno on the surface of the planet, whereas Earth is much more temperate. Outer Planets For the outer planets, the division is more distinct, with the inner two gas giants, Jupiter and Saturn being quite different in many ways from the outer two gas giants, Uranus and Neptune.

In terms of size, Saturn, with a diameter of 120,000km is not a world away from Jupiter, with a 142,000km diameter. However the other two planets, Uranus and Neptunes have diameters of 'only' 51,800km and 50,000km respectively - roughly the size of the core of Saturn or Jupiter. This large size difference is responsible for many other differences between the two groups, for instance Jupiter and Saturn both have sufficiently high internal pressures to 'squeeze' large quantities of Hydrogen into a metallic state in their cores, whilst Uranus and Neptune can produce only a tiny fraction of this amount. As a consequence, their magnetic fields are much smaller than those of Saturn and Jupiter, whose magnetic fields are generated in large part thanks to electrical currents moving through this material. Dwarf Planets This category of object is a recent creation (2006), introduced to accomodate objects orbiting the Sun which are not quite large enough to be termed 'planets', but whose size warrants them a special classification above 'asteroid' or floating object.

The criteria for a dwarf planet are that it must orbit the Sun, have enough mass such that its gravity causes it to be almost round in shape, and it must not be a satellite of another planet. The closest dwarf planet to the Sun is Ceres, which lies within the asteroid belt, then there is Pluto, which normally lies just beyond Neptune, and finally there is Eris, discovered in 2005, which normally lies beyond Pluto. To learn more about dwarf planets, see the Dwarf Planets page . Kuiper Belt The Solar System does not end with either Pluto or Eris, for as mentioned earlier there is another large 'belt' of objects orbiting the Sun, a belt covering a much greater area and containing a much greater mass than the asteroid belt. This belt is called the Kuiper Belt, and it is a fairly recent discovery - the first evidence for it was only found in the mid 1980s so we still do not know a lot about it. Most Kuiper Belt Objects, or KBOs, have not yet been seen by astronomers, and it is expected that there are a large number, perhaps greater than 40 or 50, whose size is comparable or greater than that of Pluto.

It is this reason that primarily caused the IAU to invent the 'dwarf planet' label due to the fear that otherwise there could be 40 or 50 new 'planets' to add to the list over the next few decades. So far, KBOs have been seen from distances ranging from 30AU to 50AU from the Sun (for reference, Pluto's average orbit is at about 39AU). It is estimated that the total mass of KBOs in this region is just above that of Mercury. However it is also thought that the Kuiper belt could extend to a potential distance of 1000AUs from the Sun! Exploration Many probes, satellites and rockets have been sent out to different planets and it is hoped that soon there will be manned missions to the Moon and Mars. The only manned landings so far have been in the late 1960's (69) and 70's with the Apollo missions. The first man in space was the Russian Cosmonaut Yuri Gagarin in 1961 and the first satellite was the Russian Sputnik in 1958.

The first man to have a 'spacewalk' was Cosmonaut Alexi Leonov. There have been no disasters involving the deaths of human lives in space, though in 1970 Apollo 13 experienced a few difficulties. At the moment however, the main goal the big space agencies are working towards is the International Space Station which is at present orbiting the Earth with a crew always on board. In 2004, both Europe and America made their intentions clear to press on with manned spaceflight by aiming to establish permanent manned bases on first the Moon, and then Mars, within the next half-century. It remains to be seen if this will be achieved. There are no big dangerous weapons in space, although that is set to change if the american military get their way. Click on the text above the planets below to find out more about them: Outer Planets of the Solar System: Jupiter, Saturn, Uranus, Neptune - Video & Lesson Transcript | Study.com Outer Planets of the Solar System: Jupiter, Saturn, Uranus, Neptune Watch short & fun videos Start Your Free Trial Today An error occurred trying to load this video. Try refreshing the page, or contact customer support.

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My name is Star. Welcome to Out of This World Tours! Today's unique tour of the solar system includes breathtaking views of the four outer planets, commonly called gas giants: Jupiter, Saturn, Uranus, and Neptune. We'll also visit the dwarf planet , Pluto. So hop on board my ship and come with me for a ride! It is a bit of a journey to get to our first stop. While we're traveling almost one billion kilometers, depending on where the two planets are in their orbits, let me give you some of the history of this area. Our solar system consists of eight planets, one sun, one dwarf planet, an asteroid belt, and many moons. The sun is in the middle of our solar system. Next come Mercury, Venus, Earth, and Mars. These are the small and rocky inner planets known as the terrestrial planets . After Mars, there is an asteroid belt that separates the inner planets from the outer ones. The outer planets are called the jovian planets, meaning huge gas giant. These planets in order are Jupiter, Saturn, Uranus, and Neptune. Pluto comes after Neptune, but it is no longer considered a planet. It is now a dwarf planet.

The outer planets are also called the gas giants because of their composition. Size is the most obvious difference between the inner and outer planets. The outer planets are huge! Our largest inner planet is Earth, and Earth is only 1/4 the size of the smallest outer planet, Neptune. The inner planets are rocky, sometimes all the way to their core. Gas giants are made mostly of gas with a non-gaseous core. Scientists haven't been able to discover exactly what exists in the core of a gas giant. They believe it may be solid or made of liquid metal. Hold on, we're traveling through the asteroid belt now, and the ride gets a bit bumpy. Finally, that's over. Look! Here we are at our first stop - Jupiter. Jupiter Jupiter is the first of the outer planets in order. It is also the biggest, with a mass that is more than all the other planets combined. Jupiter is so big that it often changes the trajectory of passing comets and thereby protects the inner planets from being hit by them. Jupiter spins very fast on its axis, so one day there is short, only 10 Earth hours. But its year is quite long, nearly 12 Earth years.

Look out your window and check out those colorful bands. The pattern of wide and narrow bands you see are made of whirling clouds. Jupiter also has very faint rings and at least 50 moons - probably more! Wow! What a great place to visit. Saturn The next stop on our tour is Saturn. Most people know Saturn for its rings, which are the brightest rings in the solar system. Many people don't know that Saturn is also a windy place, with winds at its equator that get up to 1100 miles per hour! Saturn also boasts 50 moons, including the largest moon in the solar system - Titan. Titan is bigger than the planet Mercury and is so large it has its own atmosphere. A day on Saturn is 10 hours 39 minutes, and a year lasts 29.4 Earth years. Alright, enough about this wonderful ringed planet. Off we go to planet number seven! Uranus Uranus looks different than the other planets, as you can see. It is tipped on its side. Scientists aren't sure why it is tilted, but they think it may have been hit by a large object, possibly a planet, a long time ago that caused it to tilt.

Another theory is that the gravity of its large moons is pulling it over to its side. Uranus was discovered only 200 years ago, and it is hard for scientists to study because it is so far from Earth. Scientists do know it has narrow, dark rings, is four times the size of Earth, and has at least 27 moons. Uranus was possibly hit by a large object, causing it to tip on its side. A day on Uranus is shorter than an Earth day at 17 hours 14 minutes, and its year lasts nearly a century at 84.3 Earth years. When they were first studying Uranus, scientists noticed that its orbit was different than what they expected. They thought this might be due to the gravity from another planet, so they started looking for one and found Neptune and Pluto. So that's where we're going next! Neptune Ladies and Gentlemen, look out your window as we arrive at Neptune. You probably know Neptune as the blue planet, but you might not know the blue color is caused by the methane gas in its atmosphere. Neptune is also the windiest planet, clocking wind speeds of up to 2000 kilometers per hour!

Scientists haven't studied Neptune much because it is so far away that it is hard to get information about it, and scientists have only once sent unmanned ships to visit it. × Uranus and Neptune The Jovian Planets: Uranus, and Neptune Uranus has a radius of 25,500 km, and a mass of 14.5 Earth's. It has a density similar to Jupiter: 1.27 gm/cm3. It also spins rapidly, and has a "day" of 17 hours and 14 minutes. This is the internal rotation period, the cloud layers rotate in about 14 hours. It orbits the Sun at an average distance of 19.2 AU (2.8 billion km), and takes 84 years to complete one trip around the Sun. It was the first planet to be discovered since antiquity, when it was found by William Herschel in 1781. Its name was chosen as Uranus was the father of Cronus (Saturn), and thus grandfather of Zeus (Jupiter). Comparison to Earth: Uranus is very strange in that its rotational axis lies very close to its orbital plane.

This means that the seasons on Uranus are very long, with 21 years that have near constant daylight ("summer"), and 21 years of almost perpetual darkness ("winter"). Here is a comparison of the obliquity of the planets in the solar system: How Uranus got to this configuration is a mystery. Presumably, a large body must have crashed-into Uranus and flipped it over (note that it spins backwards compared to the other planets--except Venus). The disk of Uranus is almost completely featureless. There appears to be a high-altitude haze that blocks our view of the cloud deck. Saturn also has this haze layer, but it is not as thick as that of Uranus (Jupiter doesn't really have this structure). The cloud tops of Uranus are very cold, 49 K! Uranus has a structure similar to Jupiter and Saturn, a big ball of hydrogen and helium gas with a rocky core about equal to one half of an Earth mass. The structure of Uranus is just like the other gas giants. The central pressure is high (8 Mbar), and with a high temperature: 5,000 K.

But both Uranus and Neptune are somewhat different from Jupiter in Saturn--they have extensive "ice mantles" : Again, this is not the ice you are familiar with---this ice is formed by high pressure (and the temperature is well above the freezing point of water on Earth): the density of this mantle is near 9 gm/cm3. It is still not completely clear whether Uranus even has a rocky core. As there are models that can explain the density that do not include such an feature. Here's the atmospheric profile: Even though Uranus is mostly featureless, it has been possible to track clouds, and thus deduce the wind speeds in its atmosphere: At the time of these observations (South pole pointing at Sun), the wind speeds were much higher closer to the pole, than near the equator (note that 100 m/s = 360 km/hr = 250 mph). By false color stretching, we can extract some details, showing that there are bands/zones like the other Jovian planets: And even spots: If you notice, the last two pictures show radically different orientations of Uranus.

In 1986, when Voyager 2 flew by Uranus, the south pole was pointed towards the Sun. But by the time of the HST picture in 2006, we were looking at the equator. Remember that Uranus has an orbital period of 84 years, thus the four seasons each last 21 years. It is 20 years between these two pictures, so our view of Uranus has changed rather dramatically! Here is a small table with times of the Uranian seasons: With the increased sunlight, Uranus has begun to have some storms and show more structure (infrared images): And another: Yes, Uranus has rings (see below). Like Jupiter and Saturn, Uranus emits more heat than it recieves from the Sun, but just barely---about the same as that emitted by the Earth. There may be a layer inside Uranus that acts to trap the internal heat, but that remains unclear.

Like the other Jovian planets, it has a magnetic field, but a very strange one: The field has a large tilt/angle with respect to the rotational poles (59 degrees), it is also not centered on the core of Uranus: Like all the Jovian planets, Uranus has a ring system, but here the rings have much less material in them as Saturn (much more than Jupiter!), and they are not quite as spectacular: Uranus now has 27 named moons (from the works of Shakespeare and Pope), several of them quite interesting in their own rite, but none of them are very large (the largest, Oberon, has a radius of 761 km, for a comparison of the satellite systems of the Jovian planets, see the Figure 11.14 in the textbook also shown here:). Identifying the moons: The Uranian moon system is the least massive of the Jovian planets. This might be tied to the strange tilt of the spin axis. If something hit Uranus, then it would have scattered the moons into the solar system, and the small set of moons we see now might have mostly formed from the result of that collision.

Here are images of the five biggest moons Oberon (ρ = 1.63 gm/cm3), Titania (ρ = 1.71 gm/cm3), Ariel (ρ = 1.66 gm/cm3), Umbriel (ρ = 1.39 gm/cm3), and Miranda (ρ = 1.20 gm/cm3), probably mostly water ice: Miranda has a dramatically fractured surface, as if it was stuck together from several large pieces. It is believed that there was enormous geological activity in Miranda's past, when it was stuck in a 3:1 resonance with Umbriel. This caused the orbit to get ellipitical, increasing the tidal heating due to the intense tidal forces exerted on it by Uranus. All of these moons are dominated by water ice, and it is possible that some of them have tenuous atmospheres created by geysers or cryovolcanoes. We had only a brief visit to Uranus 30 years ago, so much of the system remains a mystery. Neptune Neptune is much further from the Sun than Uranus (30 AU), and takes 165 years to complete one orbit.

On July 12th, 2011, Neptune completed the first full orbit since its discovery in 1846. The story of the discovery of Neptune is rather interesting, and you can read more about it here (it is a story with a bit of intrigue/lying/cheating due to the intense British-French rivalry at the time). Summarizing, however, Uranus was not behaving itself. It was not moving as one would expect, and it was proposed that there must be something out there pulling on it. Calculations were made, and the hunt was on. It was eventually discovered by Galle in Germany. Neptune has 13 moons, many fewer than any of the other Jovian planets. Neptune has 17.2 Earth masses, so it is more massive than Uranus. Note, however, that it is smaller in size than Uranus, having a radius of 24,700 km. Thus, it must have a higher density: 1.63 gm/cm3. Neptune has a rotation period of 16 hours.

Strangely, Neptune's atmosphere appears to be much more active than that of Uranus (maybe the haze layer "freezes out" at this distance, where the cloud-top temperature is colder than that of Uranus), and has several large storms similar to those on Jupiter: Close-up on high latitude, high velocity clouds: Additional activity: The "great dark spot", and the "small dark spot", and "scooter" (the white blob between the two spots--it moved around very quickly). A movie of the rotation of Neptune, made with the Hubble Space Telescope. Note that Neptune is far away, and it is hard to see detail. So, as you have noticed, the images we have presented of Uranus and Neptune show them to have a blue-green color. This is real. As we have discussed, the color of an object indicates that it is good at reflecting that color. In the case of Uranus and Neptune, their color derives from the most important absorber in their atmospheres---methane. Here is the spectrum of methane 1 : While Jupiter and Saturn certainly have lots of methane, they are warm enough that there are clouds due to ammonia, water, and sulfur molecules.

But at Uranus and Neptune it is too cold at the cloud layer to have such molecules--methane dominates the atmosphere at these levels. Thus, it is what shapes the spectra of these two planets. Recently, astronomers have created a new class of planet for Uranus and Neptune because they actually are somewhat different than Jupiter and Saturn. While we still call them "Jovian" planets, the preferred terminology is "ice giants", as inside of Uranus and Neptune there is a large region of water, methane and ammonia and is a strange form of "ice": Here's a close-up of Neptune's internal structure: [Note that Neptune's rings are clumpy, and these various clumps have been given French names.] Neptune is more massive than Uranus, and this increases its density and that means that it is slightly smaller than Uranus. Here's a graphic showing the sizes of the Jovian planets (compared to Earth): Neptune generates much more internal heat than Uranus, as it emits 2.6 times as much energy as it recieves from the Sun. This might explain why Neptune appears to have much activity in its atmosphere.

Neptune only receives 40% of the sunlight that Uranus gets, but the atmosphres of both have similar temperatures due to the internal heat emitted by Neptune. "Neptune is the farthest planet from the Sun, yet its internal energy is sufficient to drive the fastest planetary winds seen in the Solar System. Several possible explanations have been suggested, including radiogenic heating from the planet's core, conversion of methane under high pressure into hydrogen, diamond and longer hydrocarbons (the hydrogen and diamond would then rise and sink, respectively, releasing gravitational potential energy), and convection in the lower atmosphere that causes gravity waves to break above the tropopause" 2 .

Neptune has a very similar magnetic field to Uranus in that it is tilted off of the rotational axis (by 47 degrees), and also offset from the center, but even more so than Uranus: it is located one half of a radius from the center 3 : Neptune's very tenuous ring system, more similar to Jupiter's, than Saturn and Uranus: Neptune has one very large moon (of the thirteen known) named Triton , that is 2,700 km in diameter, and has a density of ρ = 2.06 gm/cm3: Triton is the only large moon in the solar system that orbits its planet "backwards" (retrograde). It also has a large inclination with respect to the equator of Neptune (157 degrees). Because of this, and the expectation that large moons form in the left over accretion disk that forms the parent planet (and thus form near the equator of the planet), it is believed that Triton is a captured "Kuiper belt" object (next class) like Pluto.

The combination of this orbital tilt, along with Neptune's own axial tilt (obliquity), Triton has seasons like those of Uranus, only longer! The internal structure of Triton: The capture of Triton probably disrupted the orbits of any small moons that were orbiting Neptune at the time--this explains the paucity of moons around Neptune. It also explains why Neptune's second biggest moon, Nereid (ρ = 1.5 gm/cm3), has the most eccentric orbit of any moon in the solar system (e = 0.75!), varying in distance from Neptune by a factor of 7! The rest of the moons are small chunks of rock (the largest of these has a mass of 0.0025 of Triton), several are associated with the rings of Neptune. Triton has a tenuous atmosphere that is mostly composed of nitrogen. A surprising discovery were these black plumes seen in the photo below that appear to be liquid nitrogen geysers. The plumes seem to be blowing in a wind from the point of origin: "Triton has a tenuous nitrogen atmosphere, with trace amounts of carbon monoxide and small amounts of methane near the surface.

Like Pluto's atmosphere (next class), the atmosphere of Triton is believed to have resulted from evaporation of nitrogen from the moon's surface. The surface temperature is at least 35.6 K (-237.6 C) because Triton's nitrogen ice is in the warmer, hexagonal crystalline state, and the phase transition between hexagonal and cubic nitrogen ice occurs at that temperature. An upper limit in the low 40s (K) can be set from vapor pressure equilibrium with nitrogen gas in Triton's atmosphere. This temperature range is colder than Pluto's average equilibrium temperature of 44 K (-229 C). Triton's surface atmospheric pressure is only about 0.017 millibar. Turbulence at Triton's surface creates a troposphere (a "weather region") rising to an altitude of 8 km. Streaks on Triton's surface left by geyser plumes suggest that the troposphere is driven by seasonal winds capable of moving material of over a micrometer in size.

Unlike other atmospheres, Triton's has no stratosphere, and instead consists of a thermosphere from 8 to 950 km above the surface, and an exosphere above that. The temperature of Triton's upper atmosphere, at 95 kelvins, is higher than the temperature at the surface due to heat deposited from space. A haze permeates most of Triton's troposphere, believed to be composed largely of hydrocarbons and nitriles created by the action of sunlight on methane. Triton's atmosphere also possesses clouds of condensed nitrogen that lie between 1 and 3 km from the surface." 3 Triton haze layer: Let's go back to that diagram for the triplepoint diagram for methane on Triton: Note that the phase diagram for nitrogen is also plotted here (in green). The surface temperature of Triton is 35 K. The observed atmospheric pressure is 0.017 millibar (= 1.7 x 10-5 bar).

Thus, you could predict that Triton would have a nitrogen atmosphere with a pressure close to 0.01 bar, as that is close to the equilibrium point between solid nitrogen and gaseous nitrogen at the surface temperature of Triton. A short movie on the Voyager spacecraft that toured the Jovian planets (launched in 1977), with Voyager 2 passing by Uranus in 1986, and Neptune in 1989. In the four or five years since this movie, the Voyagers have now left the solar system, and Voyager 1 is now officially in interstellar space ( current locations ). Voyager 2 is the only spacecraft that ever went by Uranus and Neptune, and nothing new is being planned or even talked about. Planets - Zoom Astronomy Your age on the Planets The Planets (plus the Dwarf Planet Pluto) Our solar system consists of the sun, eight planets, moons, many dwarf planets (or plutoids), an asteroid belt, comets, meteors, and others. The sun is the center of our solar system ; the planets, their moons, a belt of asteroids , comets , and other rocks and gas orbit the sun.

The eight planets that orbit the sun are (in order from the sun): Mercury , Venus , Earth , Mars , Jupiter , Saturn , Uranus , Neptune . Another large body is Pluto , now classified as a dwarf planet or plutoid. A belt of asteroids (minor planets made of rock and metal) lies between Mars and Jupiter. These objects all orbit the sun in roughly circular orbits that lie in the same plane, the ecliptic (Pluto is an exception; it has an elliptical orbit tilted over 17° from the ecliptic). Easy ways to remember the order of the planets (plus Pluto) are the mnemonics: "My Very Excellent Mother Just Sent Us Nine Pizzas" and "My Very Easy Method Just Simplifies Us Naming Planets" The first letter of each of these words represents a planet - in the correct order. The largest planet is Jupiter. It is followed by Saturn, Uranus, Neptune, Earth, Venus, Mars, Mercury, and finally, tiny Pluto (the largest of the dwarf planets). Jupiter is so big that all the other planets could fit inside it. The Inner Planets vs.

the Outer Planets The inner planets (those planets that orbit close to the sun) are quite different from the outer planets (those planets that orbit far from the sun). The inner planets are: Mercury, Venus, Earth, and Mars. They are relatively small, composed mostly of rock, and have few or no moons. The outer planets include: Jupiter, Saturn, Uranus, Neptune, and Pluto (a dwarf planet). They are mostly huge, mostly gaseous, ringed, and have many moons (again, the exception is Pluto, the dwarf planet, which is small, rocky, and has four moons). Temperatures on the Planets Generally, the farther from the Sun, the cooler the planet. Differences occur when the greenhouse effect warms a planet (like Venus) surrounded by a thick atmosphere. Density of the Planets The outer, gaseous planets are much less dense than the inner, rocky planets. The Earth is the densest planet. Saturn is the least dense planet; it would float on water. The Mass of the Planets Jupiter is by far the most massive planet; Saturn trails it. Uranus, Neptune, Earth, Venus, Mars, and Pluto are orders of magnitude less massive.

Gravitational Forces on the Planets The planet with the strongest gravitational attraction at its surface is Jupiter. Although Saturn, Uranus, and Neptune are also very massive planets, their gravitational forces are about the same as Earth. This is because the gravitational force a planet exerts upon an object at the planet's surface is proportional to its mass and to the inverse of the planet's radius squared. A Day on Each of the Planets A day is the length of time that it takes a planet to rotate on its axis (360°). A day on Earth takes almost 24 hours. The planet with the longest day is Venus ; a day on Venus takes 243 Earth days. (A day on Venus is longer than its year; a year on Venus takes only 224.7 Earth days). The planet with the shortest day is Jupiter ; a day on Jupiter only takes 9.8 Earth hours! When you observe Jupiter from Earth, you can see some of its features change. The Average Orbital Speed of the Planets As the planets orbit the Sun , they travel at different speeds.

Each planet speeds up when it is nearer the Sun and travels more slowly when it is far from the Sun (this is Kepler's Second Law of Planetary Motion ). The Planets in Our Solar System Planet (or Dwarf Planet) In 2005, a large object beyond Pluto was observed in the Kuiper belt. A few astronomers think that there might be another planet or companion star orbiting the Sun far beyond the orbit of Pluto. This distant planet/companion star may or may not exist. The hypothesized origin of this hypothetical object is that a celestial object, perhaps a hard-to-detect cool, brown dwarf star (called Nemesis ), was captured by the Sun's gravitational field. This planet is hypothesized to exist because of the unexplained clumping of some long-period comet's orbits. The orbits of these far-reaching comets seem to be affected by the gravitational pull of a distant, Sun-orbiting object. Planet Activities and Quizzes Overview of the Solar System We compare the properties of the planets to learn how the solar system formed and why the earth is such a special place.

Key points: Terrestrial and giant planets; range of properties with distance from sun and what drives them; retention of gas and atmospheres Overview of the Solar System The Planets to Scale; from the sun, the "terrestrial planets" are Mercury, Venus, Earth, and Mars, while the "giant planets" are Jupiter, Saturn, Uranus, and Neptune (Pluto was an oddity at the end*). (from ) *On August 24, 2006, the International Astronomical Union - the body that oversees such matters - voted to strip Pluto of its planet status. By official decree, we now have only eight planets. Terrestrial Planet Properties From Mercury to Mars, their distances from the sun range from about 0.4 to 1.5 times the distance of the earth from the sun, defined as an astronomical unit (AU) Mercury, Venus, Earth, Mars all have average densities greater than 3 grams/cm3 All four have rocky surfaces and rocky/metallic interiors Atmospheres vary greatly (e.g.

none for Mercury to very dense for Venus) but heavier molecules like CO2 tend to dominate over lighter ones such as H2 Only the Earth has an appreciable magnetic field (Venus and Mercury rotate too slowly, Mars apparently no longer has a liquid core) Here is a rough comparison of their interior structures. They are all variations on a common theme, with solid iron cores inside liquid iron components (except for Mars, where the interior temperature is too low to liquify this zone), surrounded by mantles of silicate rock, overlaid with rocky crusts. (from Nick Strobel's Astronomy Notes . Go to his site at for the updated and corrected version.) Distances from the sun range from about 5 AU (Jupiter) to 30 AU (Neptune) Average densities are low, similar to water: Jupiter is 1.3 grams/cm3 and Saturn is 0.7 grams/cm3 Composition similar to Sun -- (especially Jupiter and Saturn) 80% hydrogen The average speed of an atom or molecule goes as the square root of the temperature divided by the mass of the particle. If the speeds are close to the escape velocity, gases will leak away into space.

At the high temperatures at the orbit of Mercury (and its relatively small gravity), all gaseous materials escaped. For Venus and Earth, the lower temperature and larger gravity allowed heavier gases to be retained (oxygen, nitrogen) but hydrogen and helium escaped. Mars lost most of its gases because its surface gravity is small. The giant planets were out where temperatures were low enough to retain virtually all the gases. The ice line for the Solar System lies between Mars and Jupiter. It marks a major divide in planet properties because outside of it, various kinds of frozen light molecules could accumulate on a forming planet as ices (for example, frozen water captures not only oxygen, but some hydrogen). These ices ran up the mass of the planet so it had a chance to retain the light gases, even helium and hydrogen. Thus, the giant planets (around the sun AND the other stars) probably all had to form outside of the frost line. Test your understanding before going on Formation of the sun and moon, ceiling painting, St. Isaac's Cathedral, St. Petersburg (photo by G.

Rieke) StarTeach Astronomy Education you are here > Our Solar System Our Solar System Our solar system consists of various bodies of different sizes which orbit the sun. The planets are the most important of these orbiting objects, and are arrayed in this order (first is closest to the sun): Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto. The SUN, which is actually just an ordinary star, lies at the center of our solar system. What makes the sun so special to us is simply the fact that it is so close to the Earth, only 93 million (92,960,000) miles away. This distance may seem huge, but when you realize that the closest star to us besides the sun is 25.3 trillion (25,300,000,000,000) miles away, you can see how giant space really is. Besides the obvious effects the sun has on our lives, solar activity also contributes to weather patterns on Earth. For instance, between the years 1640 and 1710, there were almost no sunspots on the surface of the sun.

This coincided with a period of very cold weather on Earth known as the "Little Ice Age." Solar eclipses are particularly fascinating to watch. During a solar eclipse, the moon, the Earth, and the sun line up so that the moon lies between the sun and Earth. From our point of view on Earth, the sun is blocked completely by the moon for a few minutes. Another beautiful sight visible from Earth are the aurorae, or northern lights. These brightly colored bands usually appear in the sky in locations near the north and south poles. The bright colors occur when high-energy particles from the sun are swept into our atmosphere and make the atoms there glow. The sun gets its energy from nuclear fusion, which is a nuclear process where four hydrogen atoms are converted into one helium atom. Since the total atomic mass of four hydrogen atoms is 0.7 percent more than the atomic mass of the helium atom, there is a little bit of extra mass left over at the end of this conversion process. In his famous E = mc^2 equation, Einstein proved that mass and energy are transferable. The extra mass created in the nuclear process described above is converted into energy.

This nuclear fusion occurs in the central core of the sun, where the temperature is about 15 million degrees Fahrenheit and the pressure is 250 billion times the pressure of the Earth's atmosphere. MERCURY is the closest planet to the sun, and so it is also the fastest moving, orbiting the sun at 107,136 mph. This fast speed led the Romans to name it after Mercury, who was the speedy messenger of the Roman gods. Since Mercury is so close to the sun, it is difficult to see from Earth. It is visible only right after sunset or right before sunrise. Mercury has no moons. Nearly 4,000 of the best photographs of Mercury were taken by the Mariner 10 space mission in 1974. Those pictures show that Mercury is a small rock that resembles the moon. However, it has long cliffs and shallow craters instead of the deep craters found on the moon. At the center of the planet is a large iron core. One year on Mercury is only 88 Earth days long, while one day on Mercury is equal to 59 days on Earth!

This means that since Mercury has no atmosphere, and the sun shines continuously on the surface for 59 Earth days in a row, the daytime temperatures on Mercury can reach as high as 800 degrees Fahrenheit, under which conditions even metals such as lead would be melted! When it is nighttime on the surface, the temperature is -250 degrees Fahrenheit! VENUS is the closest planet to Earth, and is the third brightest object in our sky after the sun and moon. Although it is similar to our planet in size, mass, and density, its surface environment is quite different. The atmosphere is made up of 96% carbon dioxide and small amounts of acids. One feature of the atmosphere is the greenhouse effect, in which heat (infrared radiation) is trapped near the surface because it gets reflected back down by the clouds. Venus is therefore very hot, approximately 900 degrees Fahrenheit. The surface has been studied by the Pioneer Venus 1 and Magellan spacecrafts, which used radar to peer through the clouds and make maps. They found that most of the surface is covered by craters and hills. There are also indications that Venus had a lot of volcanic activity in the past.

The atmospheric pressure at the surface is 90 times Earth's pressure! Venus does not have any moons. Our planet, EARTH, is very unique. The environment is rich enough to support life, and many life forms thrive. Unlike any of the other planets, Earth is covered by vast quantities of water. In fact, more than 70% of the surface is under the ocean. Temperatures on Earth are very moderate compared to the other planets. Humans can live relatively comfortably at almost any location on the surface. Our only moon is a quarter the size of Earth. It rotates on its axis at exactly the same rate that it orbits the Earth, and so we always see the same side of the moon. There is no air or water on the surface of the moon, and there is also no geological activity. The moon is the only other place in the universe that people have traveled to. MARS is a rocky red planet about half the size of Earth. It orbits the sun every 1.88 Earth years, and a day on Mars lasts about as long as one on Earth does. The planet's axis of rotation is also similar to the Earth's, which means that Mars experiences seasons as well.

One unique feature is the polar ice caps, which are a combination of water ice and frozen carbon dioxide. The ice caps melt and rebuild according to the seasons, just as they do on Earth. The surface features on Mars are huge: the Valles Marinaris is a valley longer than the whole United States, and Olympus Mons is a volcano larger than Arizona and three times higher than the Hawaiian Island volcanoes. Three spacecraft have landed on Mars, and they have sent back pictures of a relatively flat surface covered with red rocks. Mars has two moons, Phobos and Deimos, which are probably asteroids captured by Mars' orbit long ago. The ASTEROID BELT lies between Mars and Jupiter. The first asteroids, or minor planets, were discovered in the very early 1800s. Eventually over 4,000 of them were found. Astronomers had calculated that another planet should lie between Mars and Jupiter, and the asteroid belt seemed to solve this problem. Asteroids are composed of raw material from the creation of our solar system. For some reason, this material did not come together to form one planet. Asteroids are the source of most meteors that have hit the Earth.

JUPITER is named after the king of the Roman gods. It is the largest planet, yet it spins very quickly on its axis- one day is about 10 Earth hours. The atmosphere is composed mainly of hydrogen and helium, although it is the small amounts of gases like ammonia and methane which make the bands on Jupiter look so colorful. It is very turbulent in the atmosphere, with huge storms frequently lasting years. The Great Red Spot is a storm which has been in existence for 300 years, and covers an area larger than Earth! Jupiter is a gas giant with a very faint ring system. The atmosphere is so thick that it is impossible to see the rocky core that is probably at the center of this huge body. There are 16 moons orbiting Jupiter. The four brightest ones- Io, Europa, Ganymede, and Callisto- are visible from Earth through a small telescope. They were first discovered in 1610 by Galileo, who was using a telescope that he built. These four moons are therefore called the Galilean satellites. SATURN is one of the most beautiful and unusual sights in the sky. It is a giant gas planet about ten times the size of Earth.

Its density is so low that it would float in a (very) big dish of water. Like Jupiter, Saturn has an atmosphere of hydrogen and helium and is often very stormy. The most unique feature of Saturn is its bright and colorful ring system, which is easily visible from Earth with a small telescope. The rings are composed of billions of small rock and ice particles which reflect the sunlight and are held together by gravity and a few of Saturn's moons. The dark separation between the two main rings is called the Cassini division. Saturn has 18 moons. The largest is Titan, which is actually larger than Mercury and Pluto. URANUS was found in 1781 by the astronomer William Herschel. Like Saturn and Jupiter, Uranus is a gas giant with a helium and hydrogen atmosphere. Uranus has a faint ring system that cannot be seen from Earth except with a very large telescope. The rings were discovered only 23 years ago, when Uranus passed in front of a star, which appeared to blink as it passed the rings. Uranus has at least 15 moons, some of which are probably captured asteroids.

NEPTUNE was discovered in 1846, after astronomers calculated where its orbit should be in order to cause strange motions in Uranus' orbit. Neptune's atmosphere is also composed of hydrogen and helium, and it has a very faint ring system that cannot be seen from Earth. For 20 years out of every 248 years (one orbit of Pluto), Neptune is actually the farthest planet from the sun. This was the case from 1979 until 1999. Pluto is now once again the last planet in the solar system. Neptune has eight moons. PLUTO is usually (see 'Neptune' above) the farthest planet from the sun. It was found in 1930. Pluto is quite different from the other gas giants in the outer reaches of the solar system. In fact, it is simply a rock composed of frozen methane. There is an on-going debate about whether Pluto should be considered a planet, since it more closely resembles an asteroid or moon. Some astronomers believe that it is an escaped moon of Neptune, and others believe that it could be the remains of a large comet.

Pluto is extremely cold, with a surface temperature of only about -360 degrees Fahrenheit. Pluto's only moon, Charon, was discovered in 1978. REFERENCES FOR THIS PAGE: Moons of Saturn, Uranus, & Neptune Moons of Saturn, Uranus, & Neptune Summary: The moons of Saturn, Uranus and Neptune range in size, though most are small compared to Jupiter's moons. Titan, the largest in this group, has an atmosphere and liquid lakes. Sections: Titan: What lies beneath the smog? Titan Unlike most other moons in our solar system, Titan has a very thick atmosphere that hides its surface from view. Chemicals in the atmosphere give Titan its reddish color. Titan's atmospheric composition: 90% nitrogen 10% mixture of argon, methane, ethane and other hydrogen compounds Despite its distance from the Sun, Titan's surface temperature is slightly warmer than it otherwise would be due to the greenhouse effect. This is the same process that traps heat on Earth. Some scientists believe Titan's current environmental status may be similar to that of Earth's before life formed.

During the formation of the solar system, Titan formed past the frost line where temperatures in the solar nebula were cold enough for methane, ammonia and water to condense into ice. Titan is composed mostly of ices, including methane and ammonia ice. Some of this ice sublimated long ago to form an atmosphere on Titan. Over billions of years, solar ultraviolet light broke apart the ammonia molecules into nitrogen and hydrogen. The lighter hydrogen escaped from the atmosphere but the heavier nitrogen molecules remained and accumulated to give Titan its nitrogen atmosphere. SHOW ME THE MATH Titan was recently visited by the Huygens probe , which landed successfully on the moon's mountainous surface after being released from the Cassini spacecraft in 2004. The Huygens probe sent back the first real images of Titan's mysterious surface, and scientists are still analyzing the data. Saturn's Medium-Size Moons (300 — 1,500 km in diameter) Saturn has 6 medium-size moons. In order of increasing distance from the planet, they are: Mimas Rhea Iapetus All are heavily cratered, indicating generally ancient surfaces, but each shows evidence of past geological activity.

The geological activity may seem surprising when we consider how small these moons are; all are considerably smaller than the Galilean moons. Enceladus, with its grooved terrain, is barely 500 km across (small enough to fit inside Colorado). In fact, the geological activity is probably a demonstration of the effectiveness of low temperature icy geology. Icy combinations of water, ammonia and methane can melt, deform and flow at remarkably low temperatures, producing a variety of volcanic and tectonic features. The surface of Enceladus shows signs of intense recent geologic activity. The Cassini spacecraft passed by the moon in 2005 and returned images showing ice geysers erupting from the surface. The material spewed out from these eruptions is a primary cause of Saturn's E-ring. MOONS OF URANUS Uranus has 27 known moons, all of which are named after characters from the writings of William Shakespeare and Alexander Pope. Uranus' medium-size moons (300 — 1,500 km in diameter) 5 medium-size moons orbit Uranus (from innermost to outermost): Miranda Titania Oberon Uranus' medium-size moons are made of ice and rock.

Like other objects that formed far from the Sun, the ice is made up of ammonia, methane and water. These moons pose several puzzles. Ariel and Umbriel are about the same size and density, but Ariel's young features indicate volcanic and tectonic activity, while Umbriel's largely cratered, uniform surface implies a lack of geologic activity. Titania and Oberon are also very close in radius, but show different levels of geologic activity. The faults on Titania indicate internal forces while Oberon's heavily cratered, icy surface shows few signs of movement. A Canyon on Miranda (click to enlarge) Voyager 2 came closest to Uranus' smallest moon, Miranda, as it passed by on its way to Neptune. The features on Miranda's surface are different from other jovian moons. With a mixture of old and new surfaces, Miranda shows relatively few craters but has a number of canyons up to 12 miles deep. Due to Miranda's odd surface features, scientists spectulate that there is an additional source of heat, such as tidal heating. The cliff walls seen in the inset are higher than those of the Grand Canyon on Earth.

Uranus' Small Moons (click to enlarge) Uranus' small moons Uranus' smallest moons are its innermost satellites. Portia heads a group of 8 inner satellites. The Portia Group consists of Bianca, Cressida, Desdemona, Juliet, Rosalind, Cupid, Belinda and Perdita. The Portian orbit is slowly decaying due to tidal deceleration and may one day either break apart to form a planetary ring or could potentially collide with Uranus. Orbits of the other satellites in the Portia Group have a similar fate. (click to enlarge) Triton: The backward moon of Neptune Voyager 2 discovered many of Neptune's moons in 1989, and took pictures revealing many previously unknown properties of Triton. Triton's icy, spherical surface reflects so much of the sunlight it receives that it is the coldest world in the solar system. Triton is the only moon in the solar system with a retrograde orbit. This, along with Triton's high inclination relative to Neptune's equator, suggests that Triton may have been captured from interplanetary space.

The surface of Triton Photos of Triton show a strange surface shaped by more than just impact cratering, although its many undisturbed craters suggest that major geologic activity has subsided. Regions on Titan's surface show evidence of past volcanic and tectonic activity. The internal heat source for Triton's geologic activity is not known, but it may involve tidal heating. The sublimation of surface ices creates a thin, hazy atmosphere on Triton made mostly of Nitrogen. The atmosphere creates wind streaks on the surface, and unknown processes pump unusual plumes of gas and particles into the atmosphere.

Question: Which planet lies between Saturn and Neptune?
Answer: Uranus